The present invention relates to detecting an obstruction in a feeding tube of a pumped fluid system which provides fluid to a patient during a pumping cycle, and automatically removing a detected clog in the feeding tube by modifying the pumping cycle for controlling the pumping of the fluid.
U.S. Pat. Nos. 4,845,487 and 4,850,807 disclose features of a feeding system to provide nutritional fluid and medication to a patient either enterally through the alimentary canal or parenterally via an intravenous catheter. Such systems are referred to herein as pumped fluid systems. The entire contents of these patents are incorporated herein by reference, and a summary thereof is presented below.
As shown in FIG. 1, a pumped fluid system for fluid control and delivery includes a reservoir 1 for storing a fluid, and a pump supply tube 2 interconnecting the reservoir 1 and a cassette 3 (described below) which is adapted to be inserted into a receiving chamber 4 within a pump-and-control housing 5. The fluid flows down the pump supply tube 2 and into the cassette 3, and is then pumped through a feeding tube 6 into the patient.
As shown in FIG. 2, the cassette 3 is preferably provided with a compressible member such as bellows 7 for drawing fluid thereinto from tube 2 as the bellows expands and for forcing a repeatable, metered volume of the fluid into the feeding tube 6 and on into the patient as the bellows contracts. The cassette 3 includes valve 8 which allows fluid to flow from tube 2 into bellows 7 and valve 9 which enables flow of fluid from bellows 7 into the feeding tube 6. Both of these valves block backflow. Valve 8 blocks backflow through tube 2 into reservoir 1, whereas valve 9 blocks backflow into bellows 7 from feeding tube 6.
As shown in FIG. 3, pump-and-control housing 5 includes a motor 10 which rotates a cam (not shown) and thereby causes a cam follower or piston 11 to compress the cassette bellows 7 (cassette 3 is not shown in FIG. 3, but the bellows 7 would be so engageable when the cassette is inserted into chamber 4) and thereby force the feeding fluid into the feeding tube 6. A pressure sensor, which can be a piezoelectric electric transducer 12, is provided between the cassette bellows 7 and the piston 11 for measuring the pressure therebetween in order to detect obstructions in the tubing.
The flow rate of fluid to the patient may be controlled by setting the pump motor 10 to an intermittent pumping mode for pulsatile flow. Intermittent pumping involves a two stroke pumping cycle whereby the pumping chamber (i.e., the cassette bellows 7) is first filled with fluid during a retraction stroke (as piston 11 is retracted and the bellows expands) and then the fluid is expelled into the feeding tube 6 and on into the patient during a compression stroke (as piston 11 is extended and the bellows contracts). The pumping cycle is provided with a timed delay at the end of the retraction stroke by stopping motor 10 for a time period sufficient to allow the pumping chamber to fill with fluid. This period of time is also adjusted by the operator in a well known manner such that the number of cycles during a given time period multiplied by the amount of fluid in the pumping chamber expelled with each compression produces a desired flow rate for providing fluid to the patient. Typical flow rates may range from 1 ml/hr. to 300 ml/hr.
As discussed by J. M. Hofstetter in "Non-Medication Induced Nasogastric Tube Occlusion: Mechanism Determination and Resolution Studies", enteral feeding systems have the tendency, over the duration of patient feeding, to form clogs in their indwelling tubes. The tubes for enteral feeding may be of a nasogastric or gastrostomy type and are generally 8 french or larger.
Medications are commonly added to the fluid from time to time during the feeding of a patient and may temporarily increase the overall viscosity of the fluid until the medication, mixed with the fluid, has been expelled from the tube into the patient.
Poiseuille's Law, which is described in the Chemical Engineer's Handbook, Fifth Edition, at pages 5-25, indicates that fluids with higher viscosity will produce higher pressures in the tube during pumping. More specifically, during the compression stroke, the pressure within the pumping chamber and feeding tube increases as fluid is forced out of the chamber and through the tube. During the retraction stroke, while the pumping chamber fills with fluid from the reservoir, the pressure in the feeding tube will decrease as the fluid flows out of it, if the feeding tube is not clogged.
Because pumped fluid systems, such as ones using enteral feeding tubes, their connecting tubes and other compliant components (such as pumping chamber and valves) which connect to the pump, are made of flexible materials and because the feeding fluid is essentially incompressible, these components of such systems enlarge in response to increased pressure during the compression stroke of pumping. This effect is magnified with increasing fluid viscosity in accordance with Poiseuille's Law. The feeding tube and other compliant components relax by returning to their normal size as fluid flows out of the feeding tube.
FIG. 4 illustrates the buildup and dissipation of pressure in the feeding tube 6 with respect to the pumping cycle during a normal state of pumping when no clogs are present in the feeding tube. Starting at point BDC' (i.e. the time when the piston rests on Bottom Dead Center of the cam rotated by motor 10), where the pumping chamber is relaxed and filled with fluid and the compression stroke is to begin, the pressure rises as the cam rotates and the pumping chamber is compressed so that fluid is forced into the feeding tube. TDC (i.e. the time when Top Dead Center is reached) is the point where the pumping chamber is fully compressed. During the retraction stroke between points TDC and BDC, fluid continues to flow out of the tube into the patient, and pressure drops to near zero. Also, fluid is drawn into the chamber during the retraction stroke. There is a timed delay at the end of the retraction stroke which occurs between points BDC and BDC' to ensure that the pumping chamber is fully filled with fluid, even for a viscous fluid, and to control flow rate.
The output amplitude of piezoelectric transducer 12 is directly related to the pressure applied thereto. More specifically, the output signal from a piezoelectric transducer is directly dependent on the rate of change of force applied thereto. If the force is constant, the output signal from the piezoelectric transducer will be zero no matter how large the force is. When the force is changed, however, the magnitude of the output signal from the piezoelectric crystal will be directly dependent on the absolute magnitude of the applied changing force. FIG. 5 shows the output of piezoelectric transducer 12 for the normal pumping cycle discussed above in relation to FIG. 4.
If piston 11 encounters more than usual resistance in compressing bellows 7, the output of piezoelectric transducer 12 will increase in amplitude. Such higher amplitude of the transducer output can be due either to the formation of an obstruction in the tube or to an increase in fluid viscosity.
With the pumping mechanisms of known pumped fluid systems it has not been possible to reliably discriminate between (1) an increase in fluid viscosity and (2) the formation of an obstruction such as a clog. As a result, it is difficult to set a fixed threshold for distinguishing increased pressure due to clogs from the increased pressure which results from normal pumping of higher viscosity fluids, particularly such as those to which medications have been added.
Conventionally, an alarm is provided for alerting a nurse or other operator that the patient is not receiving fluid due to an obstruction. When the alarm is triggered, the pump terminates its pumping mode. The nurse or other operator then follows an intervention protocol that typically includes the following measures. First, the feeding tube is examined to make certain that it is free of obstruction caused by twisting or crimping or because the patient or some other object is lying on the tubing and thereby closing it off. Then, if no such cause external to the tubing is detected, a clog is suspected and its removal is attempted by flushing the feeding tube with a syringe filled with water or other flushing fluid. Next, if flushing fails to remove the clog, a mechanical means, such as a wire with a brush attached thereto, is inserted into the tube to push the clog out the distal end of the tube into the patient. This latter procedure, which is referred to herein as "Brush Removal", is limited to gastrostomy tubing, but there are risks associated with causing a hard object to be inserted into the patient's body. Few institutions have found these risks acceptable, so adoption of this technique is very limited.
If the clog cannot be removed by any of the above described measures, the indwelling feeding tube must be replaced. This results in patient discomfort and significant cost in terms of both equipment and the professional time required to carry out the replacement procedure.
There is a class of "flushing pumps" that attempt to reduce the incidence of clogging of indwelling feeding tubes by regularly interrupting normal feeding for a brief period of time and then flushing water through the feeding tube. See, for example, the Flexiflo.RTM. Quantum.TM. Enteral Pump Operating Manual (1993) from Ross Laboratories. Such a flushing pump is intended to keep clogs from building up over time, and after the brief flushing period, normal pumping is automatically reinstated. The amount of water and frequency of flushing is adjusted such that the patient is not over-hydrated. Typically, the flushing is performed once per hour, for 11/2 minutes each time, and 25 ml of water is delivered to the patient.
This flushing flow rate is below the gravity feed rate of a typically sized (i.e., 8 french or larger) enteral feeding tube. Such a low flushing flow rate is unlikely to produce benefits that might be derived from the scouring action of forced, turbulent, higher pressure, flushing such as the effect generated by a flushing syringe connected to the feeding tube. Also, certain patients may be oversensitive to even the minimum amount of water that flushing pumps utilize, thereby precluding their use in such patients. In any event, when a clog does occur, such flushing pumps merely alert the nurse or other operator in the usual manner using an alarm. No automatic attempt is made by the flushing pump to remove clogs which have been detected.